The present invention relates to a method for detecting frame step-out in data transmission, and more particularly to the detection of frame step-out in data transmission by an encoding method to add redundant bits, such as convolutional encoding.
Frame synchronization is carried out in bit-multiplexed transmission of a plurality of low speed data signals, so that low speed data signals, obtained by separation on the receiving side, can duly correspond to those on the sending side. According to the prior art, frame synchronization needs no special means in some cases while it does require special means in other cases. In data transmission using a voice-band modem, for instance, no special means for frame synchronization, i.e. for step-out detection, is needed if n channels of the low speed data signals having 2400 bps (n is a natural number not greater than 6) are multiplexed at a multiplexing speed of 2400 Hz into a transmission signal having 14.4 Kbps, which is then modulated at a modulating speed of 2,400 bauds. More definitely, when the modulating speed is equal to the multiplexing speed, the number of the low speed data signals demodulated at the receiving side becomes equal to the number of the low speed data signals multiplexed at the transmitting side. Thus, when the demodulation clock is recovered at the receiving side by means of the PLL, the synchronization between the transmitting and receiving sides is established without specific means to correctly separate the transmission signal into the original low speed data signals. Meanwhile, where the bit rate after multiplexing is over 14.4 Kbps, for instance 19.2 Kbps, it is customary that redundant encoding is carried out at the transmitting side to improve the reliability of data transmission. In this situation, the number of multiplexed data channels is different from the number of transmitted data channels. For example, a 7-bit data signal from the 8 channels is redundant encoded to an 8-bit signal to which a data signal point is assigned in the modulator. It follows that 7.times.8=56 bits are necessary for the first channel to appear at the initial state. Therefore, the data transmission using redundant encoding requires the frame synchronization to indicate the boundary of 56 bits in the transmission signal. This frame synchronization is performed, according to the prior art, with a frame sync pulse or the like inserted into transmission data. Since a frame sync pulse is not an information datum to be transmitted, this results in wasting part of the transmission capacity which corresponds to the frame sync pulse. Meanwhile, a method to detect step-out using no such additional datum is disclosed, for instance in the U.S. Pat. No. 4,837,766. According to this U.S. patent, there are used a convolutional encoder and a data mapping table, which are not transparent to the phase revolution of 90.degree..times.n (n is an integer). In this specification, the term "transparent to a 90.degree..times.n phase revolution" with regard to a mapping table means that no decoding error occurs even if the received data suffers from a 90.degree..times.n phase revolution on a transmission line. Therefore, if the data having a phase lag of 90.degree..times.n is received its convoluted state is disordered upon decoding by a Viterbi decoder. As a result, a frame step-out is detected by the detection of the decoding error that arises whenever a frame step-out occurs. To be more specific, as illustrated in FIG. 3 of the patent, mapped data after trellis coding are successively subjected to a 90.degree..times.n phase revolution and transmitted, while decoding is performed on the receiving side after subjecting the received data to a similar phase revolution. If frame synchronization is achieved in the transmission and reception of mapped data, the data will be properly decoded. Any frame step-out that may have arisen can be detected by an error in decoded data. The frame step-out detection described in U.S. Pat. No. 4,837,766, however, is applicable only to such data mapping as would invite a data error due to a 90.degree..times.n phase revolution, but not to data mapping which is transparent to a 90.degree..times.n phase revolution shown in FIG. 3/V.32 of the CCITT Recommendation V.32, which will be referred to in further detail below.
Since data mapping which is transparent to a 90.degree..times.n phase revolution is less susceptible to decoding errors due to phase jumps or the like, it is desirable, in order to improve the reliability of data transmission as well, that such data mapping can be used.